x = 81

\(I = \cfrac{V}{R_{eq}} = \cfrac{9}{1+\cfrac{12\times4}{12+4}} = \cfrac{9}{4}\)
\(I_1 = \frac{9}{4} \times \frac{4}{16} = \frac{9}{16}\)
\(V_A - V_B = I_1 \times 8 = \frac{9}{16} \times 8 = \frac{9}{2} V\)
\(\therefore U = \frac{1}{2} \times 4 \times \frac{81}{4} \mu J\)
\(\therefore U = \frac{81}{2} \mu J\)
\(\therefore x = 81\)